The plastic deformation during flash-butt welding (FBW) and its effects on weld quality are investigated by using numerical and experimental methods. The electro-thermo-mechanical coupling model of FBW is validated by comparing the calculated temperature and plastic deformation to measured one, obtaining reasonable agreement. The calculation results reveal that a thin liquid metal film forms at the contact interface during accelerating flash stage to provide temperature conditions for upsetting. The length of liquid metal (including burning and expelled losses) is 29.7 mm for one piece pipeline tube under the given condition. The stress and strain at contact surface are both almost zero at the initial stage of upsetting due to the thin liquid metal film existing at the contact interface, and they rapidly increase to 58.0 MPa and 17.7, respectively, while the liquid metal are excluded from the contact interface between two tubes to be welded. The maximum plastic deformation is 18.1 mm at the given condition under the action of upsetting force. The experimental results illustrate that the microstructure of X65 FBW joints consists of massive ferrite, grain boundary pre-eutectic ferrite, pearlite, and widmannstatten, while the microstructure in heat-affected zone is fine ferrite and pearlite. The coarse grain size and gray spots in the butt joint severely decrease the tension strength and impact toughness.